The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
This invention generally relates to antennas and more specifically to quadrifilar antennas.
(2) Description of the Prior Art
Numerous communication networks utilize omnidirectional antenna systems to establish communications between various stations in the network. In some networks one or more stations may be mobile while others may be fixed land-based or satellite stations. Hemispherical antenna systems, i.e., antenna systems omni-directional above the azimuth and having good front-to-back ratio in elevation direction, are preferred in such applications because alternative highly directional antenna systems become difficult to apply, particularly at a mobile station that may communicate with both fixed land-based and satellite stations. In such applications it is desirable to provide an omnidirectional (in the azimuth plane) antenna system that is compact yet characterized by a wide bandwidth and a good front-to-back ratio with either horizontal or vertical polarization (in the elevation plane).
Some prior art hemispherical antenna systems use an end fed quadrifilar helix antenna for satellite communication-and a co-mounted dipole antenna for land based communications. However, each antenna has a limited bandwidth. Collectively their performances can be dependent upon antenna position relative to a ground plane. The dipole antenna has no front-to-back ratio and thus its performance can be severely degraded by heavy reflections when the antenna is mounted on a ship, particularly over low elevation angles. These co-mounted antennas also have spatial requirements that can limit their use in confined areas aboard ships or similar mobile stations. The following patents disclose helical antennas that exhibit some, but not all, of the previously described desirable characteristics.
For example, U.S. Pat. No. 5,329,287 (1994) to Strickland discloses a device for use in a helical antenna having an antenna element wound about the periphery of a dielectric support post, the post being in the form of a tube or cylinder. The device has an electrically conductive member electrically connected to one end of said antenna element. The conductive member is of any appropriate shape or configuration and is operable to increase the loading on the antenna whereby standing waves on the antenna element are reduced and a more uniform electrical current is produced along the antenna element.
U.S. Pat. Nos. 5,485,170 (1996) and 5,604,972 (1997) to McCarrick disclose a mobile satellite communications system (SMAT) mast antenna with reduced frequency scanning for mobile use in accessing stationary geosynchronous and/or geostable satellites. The antenna includes a multi-turn quadrifilar helix antenna that is fed in phase rotation at its base and is provided with a pitch and/or diameter adjustment for the helix elements, causing beam scanning in the elevation plane while remaining relatively omni-directional in azimuth. The antenna diameter and helical pitch are optimized to reduce the frequency scanning effect, and a technique is disclosed for aiming the antenna to compensate for any remaining frequency scanning effect.
U.S. Pat. No. 5,701,130 (1997) to Thill et al. discloses a self phased antenna element with a dielectric. The antenna element has two pairs of arms in a crossed relationship to transceive a signal at a resonant frequency. A dielectric is disposed adjacent an arm to obtain a self phased relationship in the arms at the resonant frequency. The arms can form crossed loops or twisted crossed loops such as a quadrifilar helix antenna element. A dielectric collar on arms of the same loop causes currents to be equally spaced from one another. The antenna size is reduced and a cross section of the antenna element appears circular without degradation of a gain pattern when the dielectric is used on a certain arm.
In U.S. Pat. No. 5,721,557 (1998) Wheeler et al. disclose a nonsquinting end-fed quadrifilar helix antenna. In essence this patent uses a limited series capacitive loading along the antenna element length. The disclosed antenna is 4 wavelengths long and is an array. Each conductor of the antenna is fed with a successively delayed phase representation of the input signal to optimize transmission characteristics. Each of the conductors is separated into a number, Z, of discrete conductor portions by Z-1 capacitive discontinuities. The addition of the capacitive discontinuities results in the formation of the antenna array. The end result of the antenna array is a quadrifilar helix antenna which is nonsquinting, that is, the antenna radiates in a given direction independently of frequency.
Quadrifilar helix antennas having a diameter of between 0.1 and 0.25 wavelengths are good candidates for satellite communications since they have overhead cardoid shaped patterns of circularly polarized signals and reasonable front-to-back ratios. However, these antennas do have pattern limitations. For a practical, useful impedance bandwidth, each antenna element must be at least three-quarters wavelength long. For example, an antenna with elements of that length and a diameter of 0.125 wavelengths can be constructed with a pitch angle of 65xc2x0. For a higher pitch angle helix, i.e., greater than 50xc2x0, impedance bandwidth increases with element length, but much more slowly than, for example, a 40xc2x0 helix which cuts in sharply near xc2xexcex and then is well matched forever. If the 65xc2x0 helix is to be well matched, e.g., near xc2xexcex its impedance bandwidth, when translated to a characteristic impedance, e.g., a feed Z0 of 50 ohms, is about 12%. If the effective length of the antenna is greater than three-quarters of a wavelength, the patterns start to multilobe and split above the horizon with the severity of the splitting in terms of the depth of the pattern nulls being determined by antenna element pitch angle. The observed nulls are less deep for sharper beam, lower pitch angle, helices. However, for any quadrifilar helix, the pattern does tend to flatten toward the horizon as frequency increases.
Stated differently, for all quadrifilar helix antennas, increasing the pitch angle broadens the pattern toward the horizon; lower pitch angles produce sharper overhead patterns. Normally the broader patterns near the horizon are desired for satellite communication so some flattening of overhead gain is permissible since the distance to the satellite is generally less overhead than near the horizon. While the impedance bandwidth can be increased by allowing the antenna elements to become longer as measured by wavelengths, this will also produce a multilobing problem above the three-quarter wavelength distance.
As described in the prior art, there exists a family of quadrifilar helices that are broadband impedance wise above a certain xe2x80x9ccut-inxe2x80x9d frequency, and thus are useful for wideband satellite communications including Demand Assigned Multiple Access (DAMA) UHF functions in the range of 240 to 320 MHz and for other satellite communications functions in the range of 320 to 410 MHz. Typically these antennas have (1) a pitch angle of the elements on the helix cylindrical surface from 50 down to roughly 20 degrees, (2) elements that are at least roughly xc2xe wavelengths long, and (3) a xe2x80x9ccut-inxe2x80x9d frequency roughly corresponding to a frequency at which a wavelength is twice the length of one turn of the antenna element. This dependence changes with pitch angle. Above the xe2x80x9ccut-inxe2x80x9d frequency, the helix has an approximately flat VSWR around 2:1 or less (about the Z0 value of the antenna). Thus the antenna is broadband impedance-wise above the cut-in frequency. The previous three dimensions translate into a helix diameter of 0.1 to 0.2 wavelengths at the cut-in frequency.
For pitch angles of approximately 30xc2x0 to 50xc2x0, such antennas provide good cardoid shaped patterns for satellite communications. Good circular polarization exists down to the horizon since the antenna is greater than 1.5 wavelengths long (2 elements constitute one array of the dual array, quadrifilar antenna) and is at least one turn. At the cut-in frequency, lower angled helices have sharper patterns. As frequency increases, patterns start to flatten overhead and spread out near the horizon and small nulls start to form overhead. For a given satellite band to be covered, a tradeoff can be chosen on how sharp the pattern is allowed to be at the bottom of the band and how much it can be spread out by the time the top of the band is reached. This tradeoff is made by choosing where the band should start relative to the cut-in frequency and the pitch angle.
For optimum front-to-back ratio performance, the bottom of the band should start at the cut-in frequency. This is because, for a given element thickness, backside radiation increases with frequency (the front-to-back ratio decreases with frequency). This decrease of front-to-back ratio with frequency limits the antenna immunity to multipath nulling effects.
Other factors that influence the front-to-back ratio include the method of feeding the antenna, the physical size of antenna elements, the dielectric loading of the antenna elements and the termination of the antenna elements. Looking first at antenna feeding, the front-to-back ratio improves when an antenna is fed in a xe2x80x9cbackfire modexe2x80x9d such that the antenna feed point is at the top of a vertically oriented antenna, as opposed to a xe2x80x9cforward fire modexe2x80x9d when the feed point is at the bottom of the antenna.
Thinner elements increase the front-to-back ratio somewhat. However, as the elements become thinner, the antenna characteristic impedance Z0, and thus input impedance to the antenna increases and introduces a requirement for impedance matching. Alternatively, lower impedances can be obtained by constructing an antenna with a partial overlap of the antenna elements to increase capacitance. However, a loss of impedance bandwidth starts to occur since such capacitance is non-radiating; that is, no radiation can occur from the overlapped areas of the antenna.
Increasing the dielectric loading of the helix elements decreases the front-to-back ratio. Wide flat elements found in many helix antennas have a pronounced loading since one side of each antenna element touches the dielectric. If the gap between adjacent elements is small, the field is strongly concentrated in the gap and any dielectric in the gap will load the antenna strongly. Quadrifilar helix antennas can terminate with open or shorted ends remote from the feed point. It has been found that antennas with open ends have a slightly higher front-to-back ratio than do antennas with shorted ends.
My above-identified pending U.S. Pat. Ser. No. 09/356,808 now U.S. Pat. No. 6,246,379 issued Jun. 12, 2001, discloses an antenna having four constant-width antenna elements wrapped about the periphery of a cylindrical support. This construction provides a broadband antenna with a bandwidth of 240 to at least 400 MHz and with an input impedance of 100 ohms, which matches the impedance of the antenna""s feed network. This antenna also exhibits a good front-to-back ratio in both open-ended and shorted configurations. In this antenna, each antenna element has a width corresponding to about 95% of the available width for that element. However, it was found that this antenna requires a tradeoff between the pattern shapes in the transmit and receive bands. It became necessary to allow patterns at lower receive frequencies to become sharper overhead than desired. At higher transmit frequencies, it became necessary to accept overhead patterns that were flatter overhead than desired. At even higher frequencies, small to moderate nulls were observed in the patterns because the element lengths were becoming long enough electrically for multilobing to begin.
Thus, there is a need for a quadrifilar helix antenna that will produce a more constant pattern shape over a range of frequencies. In particular, there is a need for an antenna that produces a stable pattern over an extended frequency band with a good impedance match over that band.
Therefore it is an object of this invention to provide a broadband unidirectional hemispherical coverage radio frequency antenna.
Another object of this invention is to provide a broadband unidirectional hemispherical coverage antenna with good front-to-back ratio over a range of frequencies.
Still another object of this invention is to provide a broadband unidirectional hemispherical coverage antenna that operates with a circular polarization and that exhibits a good front-to-back ratio.
Still another object of this invention is to provide a broadband unidirectional hemispherical coverage antenna that provides an essentially constant radiation pattern over a range of frequencies.
Yet another object of this invention is to provide a broadband unidirectional hemispherical coverage antenna in the form of a quadrifilar helix antenna that operates over a wide frequency band with essentially constant impedance and an essentially constant pattern shape.
In accordance with one aspect of this invention, a quadrifilar helix antenna comprises a cylindrical support extending along an antenna axis. A plurality of antenna elements are wrapped helically about the cylindrical support and along the antenna axis. Each of the antenna elements includes a plurality of series connected capacitors.
In accordance with another aspect of this invention, a quadrifilar helix antenna includes a cylindrical support extending along an antenna axis and a plurality of dielectric strips wrapped helically about the cylindrical support from a feed end to a remote end. A plurality of conductive elements are spaced along the opposite sides of the dielectric strip. Each conductive element on one side is offset with respect to a corresponding conductive element on the other side thereby to partially overlap with respect to at least one of the conductive elements on the other side. An overlapped area of a pair of spaced conductors constitutes a capacitor. This defines an antenna element formed as a plurality of series connected capacitors.